1. Field of the Invention
This invention relates to a rotary encoder mounted on a shaft of a rotary body to precisely measure the number of revolutions per minute or to measure the angular phase thereof.
2. Description of the Prior Art
A rotary encoder has been generally used as a means for detecting the number of revolutions per minute of a rotary body or to measure the angular phase, such as a motor.
A rotary encoder of this kind is provided as generally known with a rotary slit disc connected to a rotary shaft of a rotary body via a connecting means, such as a boss to be rotationally moved and having slits therein, and a fixed slit disc disposed in opposition to the rotary slit disc, fastened to a mounting member of the rotary body and having slits therein. In order to detect an accurate number of revolutions per minute of a rotary body by such a rotary encoder, it is necessary that the rotary encoder be assembled as the rotary slit disc and fixed slit disc are opposed to each other via a very narrow gap of, for example, 0.5 mm with the rotary slit disc and fixed slit disc mounted on the rotary shaft at an accurate concentricity.
A conventional built-in type rotary encoder consists mainly of a first unit provided with such fixed slit disc and rotary slit disc as mentioned above, and a second unit composed of parts of an electronic circuit, such as an IC mounted on a printed board. The rotary encoder is so formed that it can be assembled usually on the part of a user by mounting the first unit on a rotary shaft of a rotary body, and thereafter mounting the second unit thereon, to carry out a desired operation for detecting the number of revolutions per minute of the rotary body.
Thus, a first unit in a built-in type rotary encoder does not have a rotary body, such as a motor initially, i.e. during the manufacture thereof; a first unit is adapted to be combined with a rotary body unitarily on the part of a user. However, when such a conventional rotary encoder is combined with a rotary body on the part of a user, special tools and jigs are required for regulating (a) the concentricity of a rotary shaft of the rotary body and a fixed slit disc, (b) the concentricity of the rotary shaft and a rotary slit disc, and (c) a gap between the fixed slit disc and rotary slit disc. Therefore, it is troublesome to assemble the rotary encoder and difficult to regulate the positional relation between the parts thereof. Namely, it is difficult to assemble the rotary encoder excellently unless the encoder manufacturer has a special knowhow for the matter.